1. Field of the Invention
The subject invention relates to reproduction or readback of recorded information and, more specifically, to the equalization of signals read back from a recording medium. The subject invention also relates to methods and apparatus for the direct-current restoration of readback or other alternating signals.
2. Description of the Prior Art
If a constant current within the passband of interest is fed into the recording head of a magnetic tape recorder or into a similar recording device, and the resulting recording subsequently read back, the voltage appearing across the output of a playback head or similar readback device varies in amplitude in accordance with one of the well-known playback response curves having the familiar ascending segment, turnover portion and descending segment (see, for instance, N. M. Haynes, TRANSISTOR CIRCUITS FOR MAGNETIC RECORDING [Howard W. Sams/Bobbs-Merrill, Catalog No. MTR-1], Chapter X EQUALIZATION [hereinafter cited as "Transistor Circuits book"]).
Particular problems are presented by the descending segment of the playback response curve, representing compound losses from three basic sources: the tape or other recording medium, the head or other readback device, and the contact between recording medium and readback device. For instance, self-demagnetization of short wavelengths or high frequencies is characteristic of tape loss. Head losses are caused by hysteresis, eddy currents, and winding capacitance. Contact losses are attributable to head-to-tape or readback device-to-recording medium separation and similar effects.
The descending segment of the playback response curve covers the peak-to-bandedge roll-off region of the frequency band of interest which is different for different playback parameters. For instance, families of playback response curves in this respect may be produced by varying the coercivity of the recording medium or the gap length in the readback device. More typically, families of the playback response curve are produced by readback at different relative recording medium speeds, with an increase in recording medium speed shifting the peak-to-bandedge roll-off region or descending segment to higher frequencies.
In practice, it is often necessary to provide frequency peaking characteristics, sometimes also referred to as a "skirt", at the descending segment of the playback response curve. Because of the similarity of that frequency peaking characteristic to a resonance curve of an oscillating circuit, it has long been customary to employ LC networks in high-frequency equalization equipment (see the above mentioned Transistor Circuits book, pp. 151, 157 and 158). In practice, this has led to the need of bothersome phase and Q adjustments, to detrimental ringing effects and to other drawbacks, such as inductive hum pickup and the relative bulkiness and component expense associated with the use of inductors.
In an effort to overcome these disadvantages, high-frequency equalization circuits using a differentiator type amplifier without inductor elements have been proposed.
In this connection, it has been found that high-frequency noise occurring outside the bandwith of the system causes a significant reduction in signal-to-noise ratio with resultant increase in the system error rate. Elimination of this noise has required a separate low-pass filter stage which, in a multi-speed system, has to be switched with changing recording medium speed, leading to a physically cumbersome and expensive installation.
A further problem connected with the recovery of signals read back from a recording medium is brought about by the well-known "baseline shift" which affects particularly the reproduction of high density digital data. This baseline shift is a type of distortion present in the readback signal because of the non-direct-current response characteristic typical of magnetic tape and similar recording and playback systems. Its presence causes great difficulty in properly detecting the digital data, resulting in high bit error rates and sensitivity to various types of data patterns. Accepted methods of eliminating the problem comprise the use of low-direct-current content data coding techniques, such as the "bi-phase" and "delay modulation" codes. In practice, such codes display deficiencies in terms of maximum bit packing densities on the recording medium, so that the use of enhanced NRZ (non-return to zero) techniques which allow much higher bit packing densities has come to be preferred. Unfortunately, these higher bit packing densities tend to occur at the expense of greater direct-current content with resultant baseline drift.
In an effort to overcome the latter problem, direct-current restoration has been resorted to. The basic form of direct-current restorer is, however, not suited to the reproduction and detection of high density digital data from a recording medium. The primary reason for this is the requirement that data be detected at the midpoint of the excursion of its voltage between the peak positive and peak negative values. For simple and accurate detection, the midpoint is set for zero volts direct-current. On the other hand, the basic direct-current restorer establishes a reference near either the positive or the negative excursion of the signal.
Similar problems exist in other fields, so that the utility of the subject invention is not limited to readback technology.